Method of preparing ceramic powders using chelate precursors

ABSTRACT

Wet-chemical methods involving the use of water-soluble hydrolytically stable metal-ion chelate precursors and the use of a nonmetal-ion-containing strong base can be used in a coprecipitation procedure for the preparation of ceramic powders. Examples of the precipitants used include tetraalkylammonium hydroxides. A composition-modified barium titanate is one of the ceramic powders that can be produced. Certain metal-ion chelates can be prepared from 2-hydroxypropanoic acid and ammonium hydroxide.

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/833,609, entitled “Electrical-Energy-Storage Unit (EESU)Utilizing Ceramic and Integrated-Circuit Technologies for Replacement ofElectrochemical Batteries,” filed Apr. 12, 2001, now U.S. Pat. No.7,033,406 and naming Richard D. Weir and Carl W. Nelson as inventors.The above-referenced application is hereby incorporated by referenceherein in its entirety.

FIELD OF THE INVENTION

The present invention relates to methods for preparing ceramic powders,and particularly to wet-chemical processes using chelate precursors.

BACKGROUND OF THE INVENTION

Ceramic powders are used in the fabrication of numerous different typesof devices including specialized mechanical components, coating formechanical components, semiconductor devices, superconducting devices,device packaging, passive electronic components such as capacitors, andmore sophisticated energy storage devices. Numerous different techniquesexist for the synthesis and fabrication of ceramic powders includingsolid phase synthesis such as solid-solid diffusion, liquid phasesynthesis such as precipitation and coprecipitation, and synthesis usinggas phase reactants. Moreover, a host of related fabrication techniquescan also be used including: spray drying, spray roasting, metal organicdecomposition, freeze drying, sol-gel synthesis, melt solidification,and the like.

Various advantages of wet-chemical methods used in the preparation ofpowders for the fabrication of ceramics have been well-known since theearly 1950s. Pioneering work in this area has been done at theMassachusetts Institute of Technology, the National Bureau of Standards(now the National Institute of Standards and Technology), PhilipsResearch Laboratories, and Motorola, Inc.

Despite the advantages of wet chemical processes, the ceramics industrylargely remains reluctant to employ these techniques. Conventionalmethods for preparing ceramic powders entail mechanical mixing of drypowders of water-insoluble carbonates, oxides, and sometimes silicates,where each constituent of the ceramic composition is carefully selectedindividually. For example, if the ceramic composition has nineconstituents in solid solution, then correspondingly nine startingpowders are selected in accordance with the amount of each required forthe end product compound. The starting powders are very likely to havedifferent median particle sizes and different particle sizedistributions. In an attempt to comminute the mixture of powders to asmaller, more uniform particle size and size distribution for eachcomponent, the powder mixture is placed in a ball mill and milled forseveral hours. The milling process generates wear debris from the ballmill itself and, the debris becomes incorporated in the powder mixture.Because of the often wide disparity in particle size among the variouscommercially available starting powders (and even significant variationin particle size of the same powder from lot to lot), an optimum resultfrom ball milling rarely occurs, and a contamination-free product isnever obtained.

Moreover, additional processing steps are still required. Solid-soliddiffusion at high temperature (but below the temperature at whichsintering starts) of the ball-milled powder mixture is required to forma usable and, preferably, fully reacted homogeneous single powder. Thefiner each powder in the mixture is, the higher the particlesurface-to-volume ratio is for each. This means that there is a greatersurface area per unit weight of each powder for the solid-soliddiffusion to occur. Moreover, longer times spent at high temperature(e.g., the calcining temperature) produce a more satisfactory endproduct. Homogeneity is improved by repeating several times theball-milling and calcining steps in succession, each requiring severalhours. Of course, this increases the amount of ball-milling wear debrisadded to the powder, thereby increasing the amount of contamination inthe end ceramic product.

Accordingly, it is desirable to have improved wet-chemical processingtechniques to prepare ceramic powders for use in the fabrication ofvarious different devices and materials.

SUMMARY OF THE INVENTION

It has been discovered that wet-chemical methods involving the use ofwater-soluble hydrolytically stable metal-ion chelate precursors and theuse of a nonmetal-ion-containing strong base can be used in acoprecipitation procedure for the preparation of ceramic powders.Examples of the precipitants used include tetraalkylammonium hydroxides.A composition-modified barium titanate is one of the ceramic powdersthat can be produced. Certain metal-ion chelates can be prepared from2-hydroxypropanoic acid and ammonium hydroxide.

In one embodiment in accordance with the invention a method isdisclosed. A plurality of precursor materials are provided in solution.Each of the plurality of precursor materials in solution furthercomprises at least one constituent ionic species of a ceramic powder. Atleast one of the plurality of precursor materials in solution is achelate solution. The plurality of precursor materials are combined insolution with a precipitant solution to cause coprecipitation of theceramic powder in a combined solution. The ceramic powder is separatedfrom the combined solution.

In another embodiment in accordance with the invention, a substantiallycontaminant free ceramic powder produced by a process is disclosed. Theprocess includes: providing a plurality of precursor materials insolution, combining the plurality of precursor materials in solutionwith a nonmetal-ion-containing strong base precipitant solution to causecoprecipitation of the ceramic powder in a combined solution; andseparating the ceramic powder from the combined solution. Each of theplurality of precursor materials in solution further comprises at leastone constituent ionic species of the ceramic powder. At least one of theplurality of precursor materials in solution is a chelate solution.

The foregoing is a summary and thus contains, by necessity,simplifications, generalizations and omissions of detail; consequently,those skilled in the art will appreciate that the summary isillustrative only and is not intended to be in any way limiting. As willalso be apparent to one of skill in the art, the operations disclosedherein may be implemented in a number of ways, and such changes andmodifications may be made without departing from this invention and itsbroader aspects. Other aspects, inventive features, and advantages ofthe present invention, as defined solely by the claims, will becomeapparent in the non-limiting detailed description set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and advantagesthereof may be acquired by referring to the following description andthe accompanying drawings, in which like reference numbers indicate likefeatures.

FIG. 1 is a flow chart illustrating ceramic powder processing techniquesin accordance with the present invention.

FIG. 2 is a flow chart illustrating chelate processing techniques inaccordance with the present invention.

DETAILED DESCRIPTION

The following sets forth a detailed description of at least the bestcontemplated mode for carrying out the one or more devices and/orprocesses described herein. The description is intended to beillustrative and should not be taken to be limiting.

The processes and techniques described in the present application can beutilized to prepare numerous different types of ceramic powders, as willbe understood to those skilled in the art. Thus, although the presentapplication emphasizes the use of these processes and techniques in thefabrication of dielectric materials for use in electrical energy storagedevices (e.g., doped or composition-modified barium titanate), the sameor similar techniques and processes can be used to prepare other ceramicpowders, and those ceramic powders may find application in themanufacture of various components, devices, materials, etc.

As noted in the aforementioned '609 patent application,high-permittivity calcined composition-modified barium titanate powderscan be used to fabricate high quality dielectric devices. U.S. Pat. No.6,078,494 (hereby incorporated by reference herein in its entirety)describes examples of various doped barium titanate dielectric ceramiccompositions. More specifically, the '494 patent describes a dielectricceramic composition comprising a doped barium-calcium-zirconium-titanateof the composition(Ba_(1-α-μ-ν)A_(μ)D_(ν)Ca_(α))[Ti_(1-x-δ-μ′-ν′)Mn_(δ)A′_(μ′)D′_(ν′)Zr_(x)]_(z)O₃,where A=Ag, A′=Dy, Er, Ho, Y, Yb, or Ga; D=Nd, Pr, Sm, or Gd; D′=Nb orMo, 0.10≦x≦0.25; 0≦μ≦0.01, 0≦μ′≦0.01, 0≦ν≦0.01, 0≦ν′≦0.01, 0≦δ≦0.01, and0.995≦z≦0≦α≦0.005. These barium-calcium-zirconium-titanate compoundshave a perovskite structure of the general composition ABO₃, where therare earth metal ions Nd, Pr, Sm and Gd (having a large ion radius) arearranged at A-sites, and the rare earth metal ions Dy, Er, Ho, Y, Yb andGa (having a small ion radius) are arranged at B-sites. The perovskitematerial includes the acceptor ions Ag, Dy, Er, Ho, Y or Yb and thedonor ions Nb, Mo, Nd, Pr, Sm and Gd at lattice sites having a differentlocal symmetry. Donors and acceptors form donor-acceptor complexeswithin the lattice structure of the barium-calcium-zirconium-titanateaccording to the invention. The dielectric ceramic compositionsdescribed by the '494 patent are just some of the many types of ceramiccompositions that can be fabricated using the processes and techniquesof the present application.

In the present application, chelates are used as precursors to one ormore of the constituent components of a target ceramic powder. Ingeneral, chelation is the formation or presence of bonds (or otherattractive interactions) between two or more separate binding siteswithin the same ligand and a single central atom. A molecular entity inwhich there is chelation (and the corresponding chemical species) iscalled a chelate. The terms bidentate (or didentate), tridentate,tetradentate . . . multidentate are often used to indicate the number ofpotential binding sites of the ligand, at least two of which are used bythe ligand in forming a chelate.

For example, various wet-chemical powder preparation techniques forcomposition-modified barium titanate are described below. The methodsmake use of aqueous solutions for some or all reactants to form bycoprecipitation the desired powders. Furthermore, the approach extendsthe use of one or more chelates (preferably water-soluble or waterstable) as precursors to several of the component metal ions comprisingthe constituents of the composition-modified barium titanate. Anonmetal-ion-containing strong base, e.g., selected from amongtetraalkylammonium hydroxides, such as tetramethylammonium hydroxide[(CH₃)₄NOH] in aqueous solution is used as the precipitant for themixture of precursors in aqueous solution. The tetraalkylammoniumhydroxides, unlike conventional strong bases, e.g., sodium and potassiumhydroxides, do not introduce contamination metal ions, e.g., sodium andpotassium ions, to the end product. Note that there are numerous organiccompounds that are basic in pH, but the tetraalkylammonium hydroxides asa group are the only organic compounds that are strong bases, e.g., asstrong as common ones: NaOH and KOH, which are inorganic compound bases.

In wet-chemical methods for the preparation of ceramic powders bycoprecipitation of a mixture of precursors from solution, small amountsof precipitant will typically be included within the micropores andnanopores of the product powder. Similarly, small amounts of precipitantwill also be adsorbed onto the surface of product powder. Where strongbases such as sodium hydroxide or potassium hydroxide are used as theprecipitant, a very large amount of DI water is consumed (typically inseveral successive washings of the precipitated powder) in the attemptto rid the product of the residual precipitant. This procedure is rarelycompletely successful, and thus some residual precipitant remains.Subsequent calcining in air of the powder product converts the residualsodium or potassium hydroxide (which upon exposure to ambient air isfirst converted to the carbonate by reaction with carbon dioxide in theambient air) to the oxide, which by solid-solid diffusion becomesincorporated within the product as a constituent. For many applications,this additional constituent is an undesirable contaminant.

This unwanted result can be circumvented by the use of any of thetetraalkylammonium hydroxides as the strong base. In the examples below,tetramethylammonium hydroxide is selected for the precipitant, butvarious other tetraalkylammonium hydroxides can be used. In principle,no washing of the precipitated powder is needed to remove residualprecipitant. However, in some embodiments, a DI water washing step, orsome other washing step, is performed. Thus, a solid-solid solution ofwater-soluble hydrated and chelated metal-ion species in theirproportioned amounts is precipitated as an oxide (thecomposition-modified barium titanate) by the nonmetal-ion-containingtetramethylammonium hydroxide.

During calcination in air of the product powder, the residuals:tetramethylammonium hydroxide, tetramethylammonium nitrate,tetramethylammonium 2-hydroxypropanate, ammonium hydroxide, ammoniumnitrate, and ammonium 2-hydroxypropanate, are thermally decomposed andoxidized and thereby completely converted to gaseous products: H₂O, NH₃,CO, CO₂, N₂, O₂, N₂O, NO, and NO₂. Another advantage of the use of atetraalkylammonium hydroxide as the precipitant is the amount of DIwater required for washing is reduced or, in principle, no DI waterwashing step is needed since the residuals are completely converted togaseous products.

Preparation of the high-permittivity calcined composition-modifiedbarium titanate powder in this manner yields high purity powders withnarrow particle-size distribution. The microstructures of ceramicsformed from these calcined wet-chemical-prepared powders are uniform ingrain size and can also result in smaller grain size. Electricalproperties are improved so that higher relative permittivities andincreased dielectric breakdown strengths can be obtained. Furtherimprovement can be obtained by the elimination of voids within thesintered ceramic body with subsequent hot isostatic pressing.

In one embodiment, at least one, but not necessarily all of theprecursors are chelates. A solution of the precursors: Ba(NO₃)₂,Ca(NO₃)₂.4H₂O, Nd(NO₃)₃.6H₂O, Y(NO₃)₃.4H₂O, Mn(CH₃COO)₂.4H₂O, ZrO(NO₃)₂,and [CH₃CH(O—)COONH₄]₂Ti(OH)₂, is formed in deionized water. In thisexample the Ti chelate [CH₃CH(O—)COONH₄]₂Ti(OH)₂ is used. As needed, thesolution can be mixed and/or heated (e.g., heated to 80° C.) and is madein the proportionate amount in weight percent for each of the precursorsas shown in Table 1.

TABLE 1 Metal element Atom fraction At Wt Product Wt % Ba 0.9575 137.327131.49060 98.52855 Ca 0.0400 40.078 1.60312 1.20125 Nd 0.0025 144.2400.36060 0.27020 Total 1.0000 100.00000 Ti 0.8150 47.867 39.0116169.92390 Zr 0.1800 91.224 16.42032 29.43157 Mn 0.0025 54.93085 0.137330.24614 Y 0.0025 88.90585 0.22226 0.39839 Total 1.0000 100.00000

A separate solution of tetramethylammonium hydroxide, possibly in excessof the amount required, is made in deionized water free of dissolvedcarbon dioxide (CO₂) and heated to 80°-85° C. Table 2 illustratesexample calculations for the minimum amount of tetramethylammoniumhydroxide needed.

TABLE 2 Precursor FW Wt % Wt %/FW Mult. Mol of base Ba(NO₃)₂ 261.3448.09898 0.184048 2 0.368095 Ca(NO₃)₂•4H₂O 236.15 1.81568 0.007689 20.015377 Nd(NO₃)₃•6H₂O 438.35 0.21065 0.000481 3 0.001442 Y(NO₃)₃•4H₂O346.98 0.15300 0.000441 3 0.001323 Mn(CH₃COO)₂•4H₂O 245.08 0.108060.000441 2 0.000882 ZrO(NO₃)₂ 231.23 7.34097 0.031747 2 0.063495[CH₃CH(O—)COONH₄]₂Ti(OH)₂ 294.08 42.27266 0.143745 2 0.287491 Total100.00000 0.738105

Since the formula weight (FW) of tetramethylammonium hydroxide is 91.15,the weight of the minimum amount of tetramethylammonium hydroxide neededfor 100 g of precursor mixture is (0.738105 mol)×(91.15 g/mol)=67.278 g.

The two solutions are mixed by pumping the heated ingredient streamssimultaneously through a coaxial fluid jet mixer. A slurry of thecoprecipitated powder is produced and collected in a drown-out vessel.The coprecipitated powder is refluxed in the drown-out vessel at 90°-95°C. for 12 hr and then filtered, optionally deionized-water washed, anddried. Alternatively, the powder can be collected by centrifugalsedimentation, or some other technique. The subsequent powder iscalcined under suitable conditions, e.g., at 1050° C. in air in anappropriate silica glass (fused quartz) tray or tube.

FIG. 1 is a flow chart illustrating ceramic powder processing techniquesin accordance with the present invention. The process begins at 100. Inoperation 110, the appropriate precursor materials, e.g., chelates andother precursors, are provided in solution (110). Next a suitableprecipitant is provided (120). The two materials are then combined toform the desired ceramic powder via a coprecipitation reaction (130).Once the ceramic powder is formed, it can be separated from the solutionin which it is formed (140) using suitable separation devices andtechniques. Other post processing steps can be employed including:washing the ceramic powder (150), drying the ceramic powder (160), andcalcining the ceramic powder (170). The process terminates at 180. Theresulting ceramic powder can then be used in the fabrication of numerousdifferent devices.

In other examples, multiple chelate precursors are used in a similarprocess. In the case of Zr, various Zr compounds can be used asprecursors. As noted in the example above, oxozirconium(IV)nitrate(zirconyl nitrate) [ZrO(NO₃)₂] can be used. However, ZrO(NO₃)₂requires a relatively low pH of about 1.5, provided by an added acidsolution, e.g., nitric acid (HNO₃), to prevent hydrolysis. Analternative approach for the precursor is the use of the hydrolyticallystable chelate: zirconium(IV) bis(ammonium2-hydroxypropanato)dihydroxide [zirconium(IV) bis(ammoniumlactato)dihydroxide] {[CH₃CH(O—)COONH₄]₂Zr(OH)₂} aqueous solution, whichis stable over the pH range from 6 to 8 up to 100° C. Although thiscompound is not readily available commercially, it can be prepared fromany of the alkyl oxides of zirconium(IV). Any of thesezirconium(IV)alkyl oxides serve as an intermediate from the zirconiumtetrachloride[zirconium(IV) chloride] (ZrCl₄) source in the preparationof all other zirconium(IV) compounds. Examples of commercially availablezirconium(IV)alkyl oxides include: the ethoxide [Zr(OCH₂CH₃)₄], thepropoxide [Zr(OCH₂CH₂CH₃)₄], the isopropoxide {Zr[OCH(CH₃)₂]₄}, thebutoxide [Zr(OCH₂CH₂CH₂CH₃)₄], and the tert-butoxide {Zr[OC(CH₃)₃]₄}.

Of these examples, zirconium(IV)isopropoxide(tetra-2-propyl zirconate)is likely to be the lowest cost because of the very large volume of2-propanol (isopropyl alcohol) produced by several manufacturers. Thesealkyl oxides are all soluble in alcohols, but they all hydrolyze in thepresence of moisture. However, by reaction with 2-hydroxypropanoic acid(2-hydroxypropionic acid, lactic acid) [CH₃CH(OH)COOH], 85 wt % inaqueous solution, followed with ammonium hydroxide (NH₄OH), 28 wt %ammonia (NH₃) in water, the water-stable zirconium(IV) chelate isprepared. The other reaction product is the alcohol from which thezirconium(IV) alkyl oxide was originally made in the reaction with thezirconium tetrachloride source. This alcohol is recoverable byfractional distillation, membrane pervaporization, or the like.

The suitable water-stable titanium(IV) chelate: titanium(IV)bis(ammonium 2-hydroxypropanato)dihydroxide[titanium(IV) bis(ammoniumlactato)dihydroxide] {[CH₃CH(O—)COONH₄]₂Ti(OH)₂}, is commerciallyavailable from, for example, DuPont with trade name Tyzor® LA. It can beprepared from any of the alkyl oxides of titanium(IV). Readily availablecommercial titanium(IV) alkyl oxides include the following: themethoxide [Ti(OCH₃)₄], the ethoxide [Ti(OCH₂CH₃)₄], the propoxide[Ti(OCH₂CH₂CH₃)₄], the isopropoxide {Ti[OCH(CH₃)₂]₄}, the butoxide[Ti(OCH₂CH₂CH₂CH₃)₄], and the tert-butoxide {Ti[OC(CH₃)₃]₄}). Of these,titanium(IV) isopropoxide(tetra-2-propyl titanate) is likely to be theleast expensive. By similar preparation methods as those described abovefor the conversion of an alkyl oxide of zirconium(IV) to thewater-stable chelate, an alkyl oxide of titanium(IV) can be converted tothe water-stable titanium(IV) chelate.

Water-soluble and/or stable chelates of manganese(II), yttrium(III),lanthanum(III), neodymium(III), and several other metal ions can beprepared with the use of 2-hydroxypropanoic acid (lactic acid) andammonium hydroxide. The most convenient starting compounds arecommercially available water-insoluble carbonates of these metal ions,because they more readily react with 2-hydroxypropanoic acid aqueoussolution to form the very stable water-soluble (ammonium2-hydroxypropanato) metal-ion chelates. Water-insoluble oxides can alsobe used as starting compounds, although they are not as quicklyreactive.

For example, a manganese chelate can be produced when the manganese(II)carbonate (MnCO₃) is converted to bis(ammonium 2-hydroxypropanato)manganese(II) (i.e., ammonium manganese (II) 2-hydroxypropanate){Mn[CH₃CH(O—)COONH₄]₂}, as shown in the following reaction equations:

Similarly, an yttrium chelate can be produced by converting yttrium(III)carbonate [Y₂(CO₃)₃] to tris(ammonium 2-hydroxypropanato) yttrium(III)(i.e., ammonium yttrium(III) 2-hydroxypropanate) {Y[CH₃CH(O—)COONH₄]₃}as shown in the following reaction equations:

A lanthanum chelate can be produced by converting lanthanum(III)carbonate [La₂(CO₃)₃] to tris(ammonium 2-hydroxypropanato) lanthunm(III)(i.e., ammonium lanthanum(III) 2-hydroxypropanate){La[CH₃CH(O—)COONH₄]₃} as shown in the following reaction equations:

A neodymium chelate can be produced by converting neodymium(III)carbonate [Nd₂(CO₃)₃] to tris(ammonium 2-hydroxypropanato)neodymium(III) (i.e., ammonium neodymium(III) 2-hydroxypropanate){Nd[CH₃CH(O—)COONH₄]₃} as shown in the following reaction equations:

In general, nitrate compounds have the highest solubilities in water, asconcentration in moles per liter of solution at 20° C., i.e., molar, andmoles per 1000 grams of water at 20° C., i.e., molal, of any salt.Uniquely, there are no water-insoluble nitrates. Since the nitrate anion[(NO₃)⁻] does not interfere with the formation of the chelate, thenitrates, too, can be used as starting compounds. The nitrates arereadily available commercially. Accordingly the first reaction of2-hydroxypropanoic acid with the oxo-metal-ion and metal-ion species asindicated above are as follows:

Then with ammonium hydroxide the reaction is:

The next-step reactions with ammonium hydroxide are the same as thosegiven above.

In the preparation of the hydrolytically stable chelates, at the firststep of the reaction of either (1) the titanium(IV) and zirconium(IV)alkyl oxides, or (2) the metal-ion(II) and metal-ion(III) carbonateswith the 2-hydroxypropanoic acid aqueous solution, the more acidichydrogen ion of the carboxyl group (COOH) splits off first to form (1)the alcohol from which the alkyl oxide was made, or (2) water and carbondioxide. With addition of the weak base ammonium hydroxide, the hydrogenatom of the hydroxyl group (OH) splits off as a hydrogen ion to formwater and the ammonium ion [(NH₄)⁺] salt of the 2-hydroxypropanatechelate. The hydrogen atom of the hydroxyl group (OH) on the carbon atom(the 2-position or alpha-position) adjacent to the carbonyl group (C═O)is relatively acidic forming a hydrogen ion splitting off withsufficiently basic conditions provided by the addition of the ammoniumhydroxide aqueous solution. Additionally, the presence of the hydroxylgroup in the 2-position to the carboxylic acid group results in anincreased acidity of the latter.

FIG. 2 is a flow chart illustrating chelate processing techniques inaccordance with the present invention. The process begins at 200. Inoperation 210, the appropriate starting material, e.g., a metal alkyloxide or a metal-ion carbonate is selected. The material is selectedbased on the metal ion it will ultimately provide to a resulting ceramicpowder. Next, the starting material is reacted with an appropriatechelating agent (220). For example, the chelating agent can be providedin aqueous solution and combined with the starting material in asuitable reaction vessel. The combined solution is also reacted with asuitable weak base (230) to complete aspects of the reaction. Theprocess terminates at 240.

As a chelating agent, 2-hydroxypropanoic acid is a bidentate ligand,since it can bond to a central metal cation via both oxygen atoms of thefive-sided ring. Since the outer cage has two or three anion groups, thetotal negative charge exceeds the positive charge of the central metalcation, and the chelate is an anion with the ammonium cations [(NH₄)⁺]for charge balance. Ammonium ion salts have high water solubilities atneutral and near neutral pH conditions.

Use of hydrolytically stable chelates in this regard is extremelyversatile, even though many of the chelate precursors are not readilyavailable commercially. In particular, such chelates have applicabilityto all the metal ions of the Periodic Table except, those of Groups IAand perhaps IIA, for coprecipitation procedures in the preparation ofceramic powders. Alkaline metal ions do not form complexes and alkalineearth metal ions (Group IIA) form rather weak complexes with2-hydroxypropanoic acid.

In general all the water-soluble 2-hydroxycarboxylic acids(alpha-hydroxycarboxylic acids) form considerably stronger complexmolecular ions with most metals ions, through bidentate chelationinvolving both functional donor groups, than do the corresponding simplecarboxylic acids. This feature makes possible in aqueous solution atneutral and near neutral pH hydrolytically stable mixtures of thesechelates involving two to nearly all metal ions and oxometal ions in anymole ratio of any one to any other. Moreover, it is important to notethat the ammonium compounds: nitrates, 2-hydroxproanates, etc.,thermally decompose and oxidize away as gases, so that they do not haveto be washed away from the product precipitate. Numerous variations onthese chelate formation techniques will be known to those skilled in theart.

Table 3 illustrates an example composition modified barium titanatecompound formed using the above described chelate precursors. In thisexample, the formula weight of the resulting compound is 237.24.

TABLE 3 Precursor FW Mol Frac. Product Wt % Ba(NO₃)₂ 261.34 0.47875125.116525 44.450 Ca(NO₃)₂•4H₂O 236.15 0.02000 4.723 1.67Nd[CH₃CH(O—)COONH₄]₃ 465.57 0.00125 0.5819625 0.207[CH₃CH(O—)COONH₄]₂Ti(OH)₂ 294.08 0.40750 119.8376 42.575[CH₃CH(O—)COONH₄]₂Zr(OH)₂ 337.44 0.09000 30.36964375 10.789Mn[CH₃CH(O—)COONH₄]₂ 269.15 0.00125 0.3364375 0.119 Y[CH₃CH(O—)COONH₄]₃410.23 0.00125 0.5127875 0.182 Total 281.4779125 100.00

In one embodiment, the two ingredient streams, one containing theaqueous solution of all the metal-ion compound precursors, and the othercontaining the aqueous solution of the tetramethylammonium hydroxidestrong base, are reacted together simultaneously and continuously in afluid jet column that provides a high turbulence energy environment. Thetotal volume for the saturated or near-saturated commercially availableand specially manufactured aqueous solutions of the precursors istypically four times that of the 25 wt % tetramethylammonium hydroxideaqueous solution. There are two options in this case for the jet fluidcolumn: (1) adjust the former to a flow rate four times that of thelatter, keeping the stream velocities equal by having the applieddriving pressure to the two streams the same, but with thecross-sectional area of the nozzle of the former four times that of thelatter; and (2) dilute one volume of the latter by three volumes of DIwater, thereby lowering the concentration from 25 wt % to 6.25 wt % Withequal volumes for both streams, the nozzles are alike, the flow ratesare equal, and the applied driving pressure is the same. The amount ofliquid processed is 60 percent greater than that of the first option,however. The first has the substantial advantage of minimizing theamount of liquid handling and the usage of DI water. There is notechnical advantage in product quality of one over the other. Examplesof such fluid jet column mixing techniques are described in U.S. Pat.No. 5,087,437 (hereby incorporated by reference herein in its entirety).

In other embodiments, other techniques and devices can be used tocombine the ingredient streams such as, for example: (1) pouring onesolution in one vessel into the other solution in another vessel andusing mechanical or ultrasonic mixing, and (2) metering the solution inone vessel at some given flow rate into the other solution in anothervessel and using mechanical or ultrasonic mixing. Numerous other mixingtechniques will be known to those skilled in the art.

The resulting slurry can be refluxed as appropriate. Next, the slurry istransferred to a filtration or separation device. The separating of theprecipitate from the liquid phase and the isolation of precipitate canbe carried out using a variety of devices and techniques including:conventional filtering, vacuum filtering, centrifugal separation,sedimentation, spray drying, freeze drying, or the like. The filteredpowder can then undergo various washing, drying, and calcining steps asdesired.

The advantages of wet-chemical methods in the preparation of powders forfabricating oxide ceramics of technical significance are enlarged inscope with the use, as precursors, of hydrolytically stable chelates ofmetal ions or oxometal ions at neutral and near-neutral pH, and with theuse, as the strong-base precipitating agent such as a tetraalkylammoniumhydroxide aqueous solution. A preferred chelating agent is the verywater-soluble 2-hydroxypropanoic acid (i.e., lactic acid) followed byneutralization with the weak-base ammonium hydroxide aqueous solution,both of which are produced in high volume and are thus relatively low incost.

In the examples illustrated above, various compounds, solutions,temperature ranges, pH ranges, quantities, weights, and the like areprovided for illustration purposes. Those having skill in the art willrecognize that some or all of those parameters can be adjusted asdesired or necessary. For example, other acids can be used in place of2-hydroxypropanoic acid as a chelating agent. Alpha-hydroxycarboxylicacids having at least the same five-sided ring including the carbonylgroup and having the two oxygen atoms of the ring bonding to the centralmetal ion or oxometal ion can be used and include:

-   2-hydroxyethanoic acid (i.e., glycolic acid, hydroxyacetic acid)    [(OH)CH₂COOH];-   2-hydroxybutanedioic acid (i.e., malic acid, hydroxysuccinic acid)    [HOOCCH₂CH(OH)COOH];-   2,3-dhydroxybutanedioic acid (i.e., tartaric acid)    [HOOCCH(OH)CH(OH)COOH];-   2-hydroxy-1,2,3-propanetricarboxylic acid (i.e., citric acid)    [(OH)C(COOH)(CH₂COOH)₂];-   2-hydroxybutanoic acid [CH₃CH₂CH(OH)COOH];-   2-hydroxypentanoic acid [CH₃(CH₂)₂CH(OH)COOH]; and-   2-hydroxyhexanoic acid (i.e., 2-hydroxycaproic acid)    [CH₃(CH₂)₃CH(OH)COOH].

These water-soluble chelating agents are also useful in preparing thewater-soluble precursors for the coprecipitation procedure, but they aremore costly than lactic acid. Other water-solublealpha-hydroxycarboxylic acids can be used as will be known to thoseskilled in the art.

Although the present invention has been described with respect tospecific embodiments thereof, various changes and modifications may besuggested to one skilled in the art and it is intended that the presentinvention encompass such changes and modifications as fall within thescope of the appended claims.

1. A method of forming a composition-modified barium titanate ceramicpowder, with a formula of(Ba_(1-α-μ-ν)A_(μ)D_(ν)Ca_(α))[Ti_(1-x-δ-μ′-ν′)Mn_(δ)A′_(μ′)D′_(ν′)Zr_(x)]_(z)O₃,where A=Ag or La, A′=Dy, Er, Ho, Y, Yb, or Ga; D=Nd, Pr, Sm, or Gd;D′=Nb or Mo, 0.10≦x≦0.25; 0≦μ≦0.01, 0≦μ′≦0.01, 0≦ν≦0.01, 0≦ν′≦0.01,0≦δ≦0.01, 0.995≦z≦1, and 0≦α≦0.005 the method comprising: individuallyforming each of a plurality of water-stable constituent ion chelates,wherein each of the water-stable constituent ion chelates is formed froma constituent ionic species of the ceramic powder, ammonium hydroxide,and a chelate agent, a plurality of water-stable constituent ionchelates including ionic species zirconium, manganese, yttrium,lanthanum, and neodymium, the chelate agent comprising2-hydroxypropanoic acid or an alpha-hydroxycarboxylic acid selected fromthe group consisting of 2-hydroxyethanoic acid, 2-hydroxybutanedioicacid, 2,3-dihydroxybutanedioic acid,2-hydroxy-1,2,3-propanetricarboxylic acid, 2-hydroxybutanoic acid,2-hydroxypentanoic acid, and 2-hydroxyhexanoic acid; providing aplurality of precursor materials in an aqueous solution, a firstprecursor material of the plurality of precursor materials comprisingBa(NO₃)₂ and calcium nitrate, a second precursor comprising a stabletitanium alpha-carboxylic acid chelate, a third precursor material ofthe plurality of precursor materials comprising the plurality ofwater-stable constituent ion chelates; combining the plurality ofprecursor materials in the aqueous solution with a precipitant solutionto cause coprecipitation of primary particles of the ceramic powder in acombined solution, the precipitant solution comprisingtetraalkylammonium hydroxide, the primary particles comprising thefirst, second, and third precursors; refluxing the coprecipitatedprimary particles of the ceramic powder; separating the ceramic powderfrom the combined solution; and calcining the ceramic powder, theprimary particles of the ceramic powder comprising composition-modifiedbarium titanate having a perovskite structure.
 2. The method of claim 1wherein the constituent ionic species of the plurality of water-stableconstituent ion chelates further include an ionic specie selected fromthe group consisting of Pr, Sm, Gd, Dy, Er, Ho, Yb, Ga, Ag, Dy, Er, Ho,Nb, and Mo.
 3. The method of claim 1 wherein the plurality ofwater-stable constituent ion chelates include zirconium(IV) bis(ammonium2-hydroxypropanato)dihydroxide; bis(ammonium 2-hydroxypropanato)manganese(II); tris(ammonium 2-hydroxypropanato) yttrium(III);tris(ammonium 2-hydroxypropanato)-lanthanum(III); and tris(ammonium2-hydroxypropanato) neodymium(III).
 4. The method of claim 1 wherein thetetraalkylammonium hydroxide is tetramethylammonium hydroxide.
 5. Themethod of claim 1 wherein the combining further comprises: mixing theplurality of precursor materials in the aqueous solution and theprecipitant solution in a fluid jet column.
 6. The method of claim 5further comprising: introducing the plurality of precursor materials insolution in a first stream; and introducing the precipitant solution ina second stream.
 7. The method of claim 1 wherein the combining furthercomprises at least one of: mechanically mixing the plurality ofprecursor materials in solution and the precipitant solution; andultrasonically mixing the plurality of precursor materials in solutionand the precipitant solution.
 8. The method of claim 1 wherein theseparating the ceramic powder from the combined solution furthercomprises at least one of: filtering the ceramic powder from thecombined solution; centrifuging the combined solution; sedimenting thecombined solution; spray drying the combined solution; and freeze dryingthe combined solution.
 9. The method of claim 1 further comprising atleast one of: washing the separated ceramic powder; drying the separatedceramic powder; and sintering the separated ceramic powder.
 10. Themethod of claim 1 wherein at least one constituent ionic species isderived from a metal alkyl oxide.
 11. The method of claim 1 wherein atleast one constituent ionic species is derived from a metal ioncarbonate.
 12. The method of claim 1 wherein the composition-modifiedbarium titanate is doped barium-calcium-zirconium-titanate.
 13. A methodof forming a ceramic powder with a formula of(Ba_(1-α-μ-ν)A_(μ)D_(ν)Ca_(α))[Ti_(1-x-δ-μ′-ν′)Mn_(δ)A′_(μ′)D′_(ν′)Zr_(x)]_(z)O₃,where A=Ag or La, A′=Dy, Er, Ho, Y, Yb, or Ga; D=Nd, Pr, Sm, or Gd;D′=Nb or Mo, 0.10≦x≦0.25; 0≦μ≦0.01, 0≦μ′≦0.01, 0≦ν≦0.01, 0≦ν′≦0.01,0≦δ≦0.01, 0.995≦z≦1, and 0≦α≦005 for use in a dielectric material, themethod comprising: for each of the constituent metal species zirconium,manganese, yttrium, lanthanum, and neodymium: mixing a first solutionwith a second solution to form a third solution comprising a metal ionor oxometal ion chelate, the first solution comprising a metal ion oroxometal ion and the second solution comprising analpha-hydroxycarboxylic acid; stabilizing the metal ion or oxometal ionchelate by adding ammonium hydroxide, the stabilized metal ion oroxometal ion chelate remaining in solution; forming a fourth solutioncomprising the stabilized metal ion or oxometal ion chelate andcomprising barium nitrate, calcium nitrate, and a stabilized titaniumion chelate, the fourth solution being an aqueous solution comprisingeach of the stabilized metal ion or oxometal ion chelate that togetherinclude the constituent metal species zirconium, manganese, yttrium,lanthanum, and neodymium; precipitating primary particles of the ceramicpowder by adding a fifth solution comprising tetraalkylammoniumhydroxide, the primary particles comprising barium, calcium, titanium,and the constituent metal species; refluxing the primary particles ofthe ceramic powder; and calcining the primary particles, the calcinedprimary particles comprising composition-modified barium titanate havinga perovskite structure.
 14. The method of claim 13 wherein thetetraalkylammonium hydroxide comprises tetramethylammonium hydroxide.15. The method of claim 13 further comprising separating the primaryparticles of the ceramic powder from solution.
 16. The method of claim13 wherein refluxing the precipitated primary particles includesrefluxing at 90° C. to 95° C.
 17. The method of claim 13 wherein themetal ion or oxometal ion chelate is selected from the group consistingof zirconium(IV) bis(ammonium 2-hydroxypropanato)dihydroxide;bis(ammonium 2-hydroxypropanato) manganese(II); tris(ammonium2-hydroxypropanato) yttrium(III); tris(ammonium 2-hydroxypropanato)lanthanum(III); and tris(ammonium 2-hydroxypropanato) neodymium(III).18. The method of claim 13 wherein the alpha-hydroxycarboxylic acid isselected from the group consisting of 2-hydroxyethanoic acid,2-hydroxybutanedioic acid, 2,3-dihydroxybutanedioic acid,2-hydroxy-1,2,3-propanetricarboxylic acid, 2-hydroxybutanoic acid,2-hydroxypentanoic acid, and 2-hydroxyhexanoic acid.
 19. The method ofclaim 13 wherein the alpha-hydroxycarboxylic acid is2-hydroxy-1,2,3-propanetricarboxylic acid.
 20. The method of claim 13wherein the alpha-hydroxycarboxylic acid is 2-hydroxypropanoic acid. 21.The method of claim 13 wherein the composition-modified barium titanateis doped barium-calcium-zirconium-titanate.
 22. A method of forming aceramic powder with a formula of(Ba_(1-α-μ-ν)A_(μ)D_(ν)Ca_(α))[Ti_(1-x-δ-μ′-ν′)Mn_(δ)A′_(μ′)D′_(ν′)Zr_(x)]_(z)O₃,where A=Ag or La, A′=A′=Dy, Er, Ho, Y, Yb, or Ga; D=Nd, Pr, Sm, or Gd;D′=Nb or Mo, 0.10≦x≦0.25; 0≦μ≦0.01, 0≦μ′≦0.01, 0≦ν≦0.01, 0≦ν′≦0.01,0≦δ≦0.01, 0.995≦z≦1, and 0≦α≦005 for use in a dielectric material, themethod comprising: individually forming each of a plurality ofstabilized metal ion or oxometal ion chelates, wherein each of thestabilized metal ion or oxometal ion chelates is formed from aconstituent ionic species of the ceramic powder, ammonium hydroxide, anda chelate agent, a plurality of water-stable constituent ion chelatesincluding the ionic species zirconium, manganese, yttrium, lanthanum,and neodymium, the chelate agent comprising 2-hydroxypropanoic acid oran alpha-hydroxycarboxylic acid selected from the group consisting of2-hydroxyethanoic acid, 2-hydroxybutanedioic acid,2,3-dihydroxybutanedioic acid, 2-hydroxy-1,2,3-propanetricarboxylicacid, 2-hydroxybutanoic acid, 2-hydroxypentanoic acid, and2-hydroxyhexanoic acid; forming a first solution from barium nitrate,calcium nitrate, a stabilized titanium ion chelate, and the plurality ofstabilized metal ion or oxometal ion chelates; precipitating primaryparticles of the ceramic powder including barium, titanium and the metalor oxometal ions of the plurality of metal ion or oxometal ion chelatesby adding a second solution comprising tetraalkylammonium hydroxide;refluxing the primary particles in solution; separating the primaryparticles from solution; and calcining the primary particles, thecalcined primary particles comprising composition-modified bariumtitanate having a perovskite structure.
 23. The method of claim 22wherein the tetraalkylammonium hydroxide comprises tetramethylammoniumhydroxide.
 24. The method of claim 22 wherein refluxing the precipitatedprimary particles includes refluxing at 90° C. to 95° C.
 25. The methodof claim 22, wherein the plurality of metal ion or oxometal ion chelatesinclude zirconium(IV) bis(ammonium 2-hydroxypropanato)dihydroxide;bis(ammonium 2-hydroxypropanato) manganese(II); tris(ammonium2-hydroxypropanato) yttrium(III); tris(ammonium 2-hydroxypropanato)lanthanum(III); and tris(ammonium 2-hydroxypropanato) neodymium(III).26. The method of claim 22 wherein the composition-modified bariumtitanate is doped barium-calcium-zirconium-titanate.
 27. A method offorming a ceramic powder with a formula of(Ba_(1-α-μ-ν)A_(μ)D_(ν)Ca_(α))[Ti_(1-x-δ-μ′-ν′)Mn_(δ)A′_(μ′)D′_(ν′)Zr_(x)]_(z)O₃,where A=Ag or La, A′=A′=Dy, Er, Ho, Y, Yb, or Ga; D=Nd, Pr, Sm, or Gd;D′=Nb or Mo, 0.10≦x≦0.25; 0≦μ≦0.01, 0≦μ′≦0.01, 0≦ν≦0.01, 0≦ν′≦0.01,0≦δ≦0.01, 0.995≦z≦1, and 0≦α≦005 for use in a dielectric material, themethod comprising: forming a first solution from barium nitrate, calciumnitrate, a stabilized titanium ion chelate, and a plurality ofstabilized metal ion or oxometal ion chelates, each of the stabilizedmetal ion or oxometal ion chelates is formed from a constituent ionicspecies of the ceramic powder, a hydroxide, and a chelate agent, aplurality of water-stable constituent ion chelates including the ionicspecies zirconium, manganese, yttrium, lanthanum, and neodymium, thechelate agent comprising 2-hydroxypropanoic acid or analpha-hydroxycarboxylic acid selected from the group consisting of2-hydroxyethanoic acid, 2-hydroxybutanedioic acid,2,3-dihydroxybutanedioic acid, 2-hydroxy-1,2,3-propanetricarboxylicacid, 2-hydroxybutanoic acid, 2-hydroxypentanoic acid, and2-hydroxyhexanoic acid; precipitating primary particles of the ceramicpowder, the particles including barium, titanium and the metal oroxometal ions of the plurality of metal ion or oxometal ion chelates byadding a second solution comprising tetraalkylammonium hydroxide;refluxing the primary particles in solution; separating the primaryparticles from solution; and calcining the primary particles, thecalcined primary particles being composition-modified barium titanatehaving a perovskite structure.
 28. The method of claim 27 wherein thecomposition-modified barium titanate is dopedbarium-calcium-zirconium-titanate.